The ultimate performance characteristic of a CCD imager is its modulation transfer function (MTF). The effective MTF of a CCD imaging system is dependent upon many parameters of the components of the imaging and signal processing paths. For electronic imaging application, CCD imagers have three inherent MTF performance limiting characteristics: aperture MTF, charge transfer MTF and diffusion MTF. The aperture MTF is a function of the cell design and aperture size and can be calculated easily. The charge transfer MTF is a function of the charge transfer efficiency (CTE) and the number of transfers the image signal will see. The CTE of a device can be both design, process and defect limited. Diffusion MTF is also a function of the cell design and size, but is dependent upon the processing, including starting material, gettering as well as oxide and doping layer thicknesses. The variability of both CTE and diffusion parameters with respect to processing leads one to test for them on a recurring basis, especially in product manufacturing.
CTE is typically measured by the inclusion of a "fill and spill" injection circuit incorporated on one end of the CCD shift register. One such circuit is described in the article of M. F. Tompsett, entitled "Surface Potential Equilibration Method of Setting Charge in Charge Coupled Devices", published in IEEE Transactions on Electron Devices, Vol. ED-22, No. 16, June 1975, pg. 305. This method of testing for CTE requires an electrical input pulse applied to an injection diode, and the level of charge metered into the device is determined by the difference between two adjacent electrodes. This testing method requires three interconnects, the generation of a controlled pulse for the diode, and the adjusting of the levels on the electrodes accordingly for the amount of desired injected signal. Any variation in the thresholds of the electrodes from device to device requires re-adjustment of the potential difference in the ratio of the potentials of the electrodes, and perhaps adjustment of the diode pulse level, to recover a signal of similar amplitude. Thus, automated testing in a manufacturing mode can be cumbersome if many devices are to be evaluated.
A testing method which is better suited for manufacturing is described in an article of S. P. Emmons et al., entitled "A Low Noise Input With Reduced Sensitivity to Threshold Voltage", published in Technical Digest of IEDM, Washington, D.C., December 1974, pg. 233. This method compensates for the threshold variations in the charge metering gates. The circuit for this test method includes a common electrode for charging and discharging capacitance associated with a floating diffusion. A pair of switches couple the potential of the common electrode to either V.sub.sig or V.sub.ref, while a third switch is coupled with an injection diode and provides for the charging current. While the threshold tolerance of this structure looks attractive, it comes with the added expense of two additional clocked control signals and an additional DC bias. Also, the on-chip circuitry translates into added area for local interconnect and bondpads, as well as additional package pins, all of which require circuit board space.